Glycosylphosphatidylinositol-anchored surface molecules of Trypanosoma congolense insect forms are developmentally regulated in the tsetse fly.

Procyclic culture forms of Trypanosoma congolense have been shown to express a glutamic acid/alanine-rich protein (GARP) on their surface. By labelling T. congolense procyclic culture forms with glycosylphosphatidylinositol (GPI) precursors, we show that GARP is bound to the membrane by a GPI anchor and demonstrate the presence of two additional GPI-anchored surface molecules of 24-34 and 58 kDa that are abundantly expressed. The 24-34 kDa molecule, which is recognised by monoclonal antibodies that bind to the surface of living trypanosomes, is resistant to proteolysis, suggesting that it consists (predominantly) of non-proteinaceous material. We have therefore named it protease-resistant surface molecule (PRS). In common with the EP and GPEET procyclins of Trypanosoma brucei, the relative expression of the T. congolense GPI-anchored molecules changes during parasite development in the tsetse fly. PRS is abundantly expressed by procyclic trypanosomes in the midgut shortly after infection, but is downregulated in established midgut forms and completely absent from the epimastigote form in the proboscis. In contrast, GARP is downregulated in parasites in the tsetse fly midgut, but upregulated in the epimastigote form. Unexpectedly, 14 days post-infection, procyclic forms frequently are negative for both PRS and GARP, suggesting that they might be expressing another stage-specific surface antigen at this point in the life cycle.